Aqueous solutions of Urea Ammonium Nitrate (“UAN”) are commonly used as agricultural fertilizer. However, these solutions, typically 50%-80% UAN by weight (20% to 50% water), are very corrosive to metals, particularly ferrous containing metals. Manufacturers of UAN solutions have tried a number of methods to resolve the corrosion problem.
The production of UAN solutions can be by batch or continuous process. Urea and Ammonium Nitrate solutions are blended with water and pH adjusted, sometimes using ammonia. During production or post-production, efforts are made to defeat or limit the corrosive effects of the UAN solution. Some manufacturers use filmers, that act to coat the metal coming into contact with the UAN solution. Filmers are quickly depleted, however, as the UAN solution comes into contact with additional metal surfaces. Filmers can also result in more active corrosion at localized or focused sites, where the filmer fails to fully coat a metal surface. Filmers also are known to cause problems with foaming and sludge formation, additional undesirable side effects of the use of filmers.
Manufacturers have used various additives as corrosion inhibitors. For example, see U.S. Pat. No. 5,376,159 where molybdate, by itself, is used as a corrosion inhibitor. A variety of other materials have been used as corrosion inhibitors. For example in U.S. Pat. No. 3,024,100, a variety of flourosilicates in combination with molybdate is disclosed as a corrosion inhibitor for UAN.